The present invention is directed to a method for infeeding and outfeeding light into and out of a light waveguide provided with a resilient coating with a device having at least one flexional coupler with which a pressure is exerted onto the coating of the light waveguide in order to produce a curved contour in the waveguide for outcoupling light therefrom which outfed light is evaluated in a measuring instrument.
Measuring methods wherein flexional couplers are employed are disclosed, for example, in allowed U.S. patent application Ser. No. 07/394,114, filed Aug. 15, 1989, which issued as U.S. Pat. No. 5,078,489 on Jan. 7, 1992, whose disclosure is incorporated herein by reference thereto, and which claims priority from German Application 38 28 604, and also in U.S. Pat. No. 4,652,123, whose disclosure is incorporated herein by reference thereto. As presented in detail in these two references, particularly exact results can be achieved when work is carried out with two transmitters and two receivers and a plurality of measurements are successively implemented proceeding from both sides of the unit under test.
This and all similar measuring methods are based on the measuring principle that the coupling of the receiver means or receivers and of the transmitter or transmitters occurs with at least one so-called flexional coupler, whereby the light waveguide experiences a deformation in the measuring region. The light emerging in the deformation region is acquired by the receiver and is offered to the measuring equipment for evaluation. The light is picked up by a rigidly arranged light receiver, for example a photodetector, and is offered for evaluation. At the transmitter, the deformed region of the light waveguide is situated in the radiation field of a light source. The amount of light which is coupled in or, respectively, coupled out is dependent on the spatial allocation of the light waveguide proceeding bent in the coupling region to the light receiver and is also dependent on the refractive index condition and on the geometry of the optical boundary surface in the flexional coupler insofar as no matching of refractive index, such as by immersion or elastic material, occurs. In and of itself, the measuring instrument is operational immediately after the mechanical actuation of the flexional coupler, for example after the closing of a cover, and the measurements could be immediately undertaken.
It has been shown, however, that the values obtained become all the more imprecise when the earlier measurement is undertaken after the bending of the light waveguide into the coupling position. Investigations have lead to the conclusion that a chronological variation of the infeed or, respectively, outfeed efficiency apparently occurs, namely in that a deformation, for example a flowing of the coating of the light waveguide, occurs at the coating of the light waveguide under the pressure of the element, for example an arbor, bending beam or pin, that effects the mechanical bending of the light waveguide and that this mechanical modification, however slight, modifies the position of the deformed region relative to the light source or, respectively, relative to the light receiver, in one instance, and in another instance, influences the geometry of the optical boundary surface. One could, in fact, eliminate such error influences in that one waits until the deformation of the coating has ceased because the final condition of the radiation field in the direction toward the photodetector is reached and a chronological change of the measured signals can no longer be identified, given a constant transmission signal.